[mrein3]

They say a picture is worth a thousand words so here goes.

I marked up that template because there seems to be some confusion with it.

The upper hole for the heater core in the non-ac trucks and the upper hole for the heater core in ac trucks line up. The other hole you need to make is marked in the center with a dimple from the factory. To make that hole I suggest you get a "Greenley" punch aka a sheetmetal hole punch.

To make the piece you need to weld in is easy if you have the factory box to bolt on to the outside of the firewall. Just make a trace on the back of that box. Then make a trace on the non-ac firewall. Then use the hole for the heater core that is in both firewalls and line up your two tracings. Now you can draw on one of them the shape of the "J" looking piece of metal you need to make. Cut it out and test the fit on the firewall. Now cut out a piece of sheetmetal and check it once again for fit.

Once it fits nicely grind off the paint around where you are going to weld and weld it in place.

Paint.

Install the rest of the AC stuff.